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  • The use of a venturi and combustion blower fan to mix primary air prior to cumbustion

     olivero updated 7 months ago 3 Members · 7 Posts
  • olivero - Florida

    November 1, 2019 at 4:44 pm

    I have a question on something I’ve run into specifically with Cleveland & Convotherm.

    Unlike a grill or a griddle where you turn the knob and it’s either on or off, cleveland uses in their KGL kettles and SGL tilt skillets as well as their Convotherm units what’s called a “Zero gas pressure system” that’s what the first tech support guy I spoke to from Cleveland called it and it stuck with me.

    The idea is you draw in the air and the gas using the fan, not relying on the pressure of the gas to give you a preset or correct mix but the fan actually controls it and it can make it hotter or colder depending on how fast it spins.

    The C4 model has a feature where it seems like it’ll run slower at times and faster at other times depending on how much heat it has to catch up.

    So here’s my question, how can you tell if the flame is proper, you can’t really see it and the only way to adjust it would be to either regulate fan speed or make some larger changes or open the orifice in the gas valve to increase gas flow.

    Doing exhaust measurements perhaps?

  • fixbear - ADK NY

    November 1, 2019 at 6:42 pm

    Exhaust gas analisis is the only way to go.  Even if you can see the flame.  But what he is calling zero pressure is not.  It’s actually 14.7 pounds or 1 bar or 408 inches of water . When you have a blower on the exhaust side  it is using that 14.7 lb’s to push in air to a lower pressure.  Question is what is the gas pressure. Though we call it a 1/2 lb, it’s actually approximate 15.2 lb’s. The regulator diaphragm senses atmospheric pressure. The Atmospheric pressure is pushing down on all of us. Why your Manometer will read 5 to7 inches water column on  natural gas. But the real pressure is somewhere around  413 inches of water.  Depending on weather conditions  If your in the eye of a hurricane, the atmospheric pressure can be down to 29 some odd inches of mercury.  30 inches of mercury is about 34 ft of water.

  • fixbear - ADK NY

    November 1, 2019 at 7:22 pm

    The burner systems you are referring to must have not only a variable negative pressure in the combustion chamber, but also a variable gas pressure to match.  A normal venturi will have a 10 degree gas cone from the nozzle and a matching 10 degree or less forcing cone that mixes the incoming air.  I a simple venturi the in out ratio is one to one. Or 50%.  But fuel gases vary from a low (hydrogen) of 1 to 4 tto a high of 17%. Natural gas is about 9.5 lbs air to one pound gas.  Propane is 23.8 ponds air to one pound gas. So the design of the venturi and it’s air restriction is important to good combustion. As well as the diameter and angle of the forcing cone.

    I know there is a lot of monoxide detectors out there. Last I knew they were all over $1500.  Myself, I still use the old chem absortion unit.  No drift and always gives me a good reading..

  • ectofix - Nashville

    November 2, 2019 at 8:06 am

    I can only chime due to similarities between Convotherm and Rational. 
    As you’ve mentioned, a Rational oven’s operating gas pressure can’t be measured with a manometer.  So, their tech manual delineates a procedure for adjusting gas flow by measuring CO2 and CO at the exhaust flue by using a combustion analyzer.  From my memory (I have to read up everytime I do this), the idea is to establish a target CO2 reading (listed in their manual) that represents a burner operating at optimum efficiency.  As such, CO would be at a minimum.  There’s more to it than that, but that’s the idea.
    As for any affects of the combustion blower’s variable-speeds in my Rational ovens, once I’ve properly set up the burner’s combustion rate at MAX blower speed (again, per their manual), there’s a flexible draft tube connected between the blower and the regulator that (I think) provides feedback to the gas valve so it can vary the gas flow based upon the varying air flow.

    There’s much to learn about combustion analysis, but I lack the experience to offer much more off the top of my head.  Indeed, I’ve worked on gas cooking equipment for twenty years and I’ve only had to have a combustion analyzer for just THESE ovens.  Yet, I’ve often wondered if furthering my knowledge on combustion analysis could serve as a means to fine tune other equipment…such as the setup in a KGL kettle.  I don’t recall Cleveland ever requiring that, so I’m just saying.
    I think folks in the HVAC trades are more up-to-speed on the importance of combustion analysis than guys like me just doing hot side work.  Heck, HVAC techs take it another step further by “clocking a meter” to gain the MOST optimum performance from a gas furnace.  Jim Bergmann has some videos about that on YouTube.

  • olivero - Florida

    November 3, 2019 at 9:01 am

    Good to know, thank you guys for chiming in.

    What i’ve seen on Convotherm is you can actually adjust the gas to air ratio on the gas valve which can give you a higher or lower CO2 rating when measure with a flue analyzer, I’ve done the analyzer work on both my units as we have one for our industrial boilers.

    It’s interesting how it all works, you can barely see the flame but it makes a big difference from what I understand as to how the flame treats everything, get it set up wrong and it can wreak havoc in no time.

    The gas pressure acceptable range is fairly wide, it’s 7″-11″ W.C if I remember correctly, where normally 5.5 or so is normal for other equipment.

    I always wondered why he called it that, the guy I spoke to seemed completely disinterested in the gas pressures.

    • fixbear - ADK NY

      November 3, 2019 at 1:02 pm

      I believe that what you are adjusting is the tension on the regulator sensing diaphragm for the proper ratio. Pressure would mean nothing here due to the varying firing rate as the air pressure varied. If you have one going out of spec, look for the feedback method. Usually burner pressure line to the spring side of the fuel gas regulating valve.It can also be done with electronic controls and a servo on the regulator.  But I don’t like that method as it has more lag time unless the regulator has to be a long way from the burner draft motor.

      • olivero - Florida

        November 3, 2019 at 3:10 pm

        It’s very possible, it feels like it’s not a tight adjustment so probably what you are saying.

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